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WHAT HAS CHANGED

As of 2:45 PM, winter weather advisory for lake effect snow was issued for southern Herkimer County in collaboration with WFO BGM based on a report of 5" in north central Oneida County. Despite this headline issuance, previous forecast thinking remains largely unchanged, as we will continue to message the potential for up to 4-5" of snow where the lake effect band remains most persistent. This will likely be just north of the Thruway. Otherwise, previous forecast remains on track with more details below...

Previous (1"30 PM EST)...Minor adjustments made to PoPs and snowfall amounts through this evening with the lake effect snow. Increasing confidence for most areas to see light snow accumulations late Monday night and/or Tuesday as guidance is coming into better agreement on the track of this clipper system. Will also begin mentioning potential for gusty (but sub- advisory-level) winds behind this clipper system Tuesday night into Wednesday.

KEY MESSAGES

1. In the wake of a cold frontal passage, region-wide wind gusts up to 35-40 mph are expected this afternoon and evening, with up to 2- 4" of lake effect snow in the southwestern Adirondacks and western Mohawk Valley.

2. There is moderate to high confidence in the potential for a light to locally moderate snowfall late Monday night through Tuesday, which could impact both the morning and evening commutes as well as any pre-holiday travel.

3. Temperatures continue to trend near to slightly above normal for the second half of the week with increasing chances for precipitation Friday into Saturday, although confidence is very low in precipitation type and amounts right now.

DISCUSSION

KEY MESSAGE 1...

Current sfc analysis as of 1:30 PM EST shows a 1002 mb low located over far southeastern Quebec, with its trailing cold front now tracking through the Mid Hudson Valley and western CT. There is very little moisture ahead of this front, so earlier snow showers have diminished as the front continues its southeastward trek.

Behind the front, however, a cold advection regime over the Great Lakes is allowing for a band of lake effect snow off of Lake Ontario. With lake temperatures of around +6C and 850 mb temperatures dropping to around -14C this afternoon, there will be sufficient lake-induced instability for the band to extend well inland, especially given the expected multi-lake connection, 30-40kt winds at 850mb, and inversion heights around 750 mb. With low-level flow trajectories currently out of the west, the band is pushing into the southern ADKS, north of the Thruway. Snow showers with this band extend all the way into southern VT and the northern Berkshires. As we head through the afternoon, low- level flow trajectories veer to around 290 degrees, which will help push the band further south into the Mohawk Valley, which is when snow will likely move over the Thruway. As this happens, the Capital District could get in on some of the snow showers as well. As we head into tonight (after 03z), the low-level flow continues to veer more to the NW. This will help push the band further south of our CWA, while lowering inversion heights, weakening winds, and more directional shear due to high pressure building into the Great Lakes will all help to reduce the inland extent of the band as well. Within the band itself, snowfall rates of up to around 1" per hour and gusty winds leading to blowing snow and reduced visibility will make for difficult travel conditions this afternoon and evening.

By the time snow stops, up to 3-5"" of snow will be possible in the southwestern ADKs and Mohawk Valley. While residence time for the band over any one location will be relatively short, the we ultimately decided to issue an advisory as of 2:45 PM EST per collaboration with WFO BGM based on reports they have received in Oneida County so far and the fact that the band looks quite impressive on radar. Lower snowfall amounts are expected elsewhere, with portions of the southern Greens, northern Berkshires, Capital District south of Saratoga Springs, and northern Catskills seeing a coating up to a half inch or so of snow. Late tonight or tomorrow morning, the band may drift back north with renewed snow shower activity over the western ADKs as a warm front lifting northwards through the region allows winds to back more to the west. However, any additional accumulations will be minor with warming 850 mb temps and less instability compared to today. The rest of the region should remain dry tonight and tomorrow with high pressure and subsidence building over the region.

With a tight pressure gradient between the sfc low to our northeast and sfc high building into the Great Lakes, winds atop the boundary layer will be at 35-45kt this afternoon and early evening. With decent BL mixing, we are expecting gusts of up to 35-40 mph to get mixed down to the surface, with the strongest winds in the typical W/NW channeled flow areas of the Mohawk Valley, Capital District, and Berkshires. We ended up increasing wind gusts by 5-10 kt from the NBM in collaboration with neighboring offices. While these gusts are expected to remain below advisory criteria, a few instances of downed trees/limbs leading to power outages can't be ruled out, as well as blowing and drifting of any new lake effect snow that falls this afternoon and evening. Winds and gusts should diminish overnight with the sfc high building in from the west.

KEY MESSAGE 2...

Late Monday night or Tuesday morning, an upper shortwave and associated sfc low embedded within fast NW flow aloft will track into southern Canada. As the system's warm front lifts northwards through the region, warm advection and isentropic lift will result in snow overspreading the region from southwest to northeast late Monday night or early Tuesday AM. There remains some uncertainty in the exact timing, but overall guidance has come to a better consensus on the storm track. Therefore, confidence is increasing in a mid-level FGEN band and additional forcing from right entrance region of the upper jet helping keep precip mainly snow through at least 18z, even with sfc temperatures warming to near freezing for valley areas. However, in the afternoon or evening, a mid-level dry slot will punch into the region, and the loss of cloud ice and better forcing moving off the E/NE may allow for a mix with or change over to rain/drizzle, especially for valley areas. Confidence is increasing for a general 1-3" for most areas, and up to 4-6" for some high terrain areas, especially the ADKs. Winter weather advisories will likely be needed for the ADKs, and possibly other high terrain areas for this timeframe. While lower elevation will likely see sub-advisory level snowfall amounts, the Tuesday AM commute and possibly the evening commute could still be slick, as well as for any pre-holiday travel during the day Tuesday.

While a lull in snow is expected in the afternoon and evening, the upper trough axis moves through Tuesday night with a sfc cold front, both of which will provide renewed forcing for ascent, as will the left exit region of an approaching NW to SE oriented upper jet streak. The best chance for additional precip looks to be in the ADKs due to lake enhancement and in the southern Greens due to upslope enhancement. These areas could see up to a couple additional inches of snow Tuesday night. Not expecting much additional precip for the valley areas, but with a secondary sfc low developing off the New England coast helping to advect colder air from the north into the region, any additional precip that does fall would likely be in the form of snow. Winds also turn gusty again behind this departing system Tuesday night into Wednesday; we bumped up wind gusts into the 25-35 mph range in collaboration with neighboring offices.

KEY MESSAGE 3...

High pressure at the surface and ridging aloft build over the region Wednesday into Thursday, resulting in large-scale subsidence and tranquil weather conditions. A weak upper shortwave is expected to slide to the southwest of our region on Christmas, but should stay far enough away that our area remains dry. A better chance for precipitation will come Friday into early next weekend as upper troughing and an associated sfc low over southern Canada or the Great Lakes move eastwards. There is still significant uncertainty in timing, amount, and type of precip given uncertainty in the upper flow evolution and the potential complex interaction of the various shortwaves embedded with the mean flow. However, with high pressure to the N/NE, some wintry precip may be possible, with the best chance for northern zones and high terrain areas, while valley areas have a better chance of seeing more rain. With ridging aloft, temperatures Wednesday and Christmas day will trend towards near or even slightly above normal. While we stuck near the NBM for temperatures Friday and Saturday given the high degree of uncertainty, temperatures to end the forecast period will ultimately be determined by the track of the aforementioned sfc low.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Through 00z Tuesday...Lake effect snow showers will remain in the vicinity of KALB/KPSF over the next few hours before shifting south by midnight. Conditions will remain primarily VFR in snow showers, though brief drops in ceilings to near MVFR levels will be possible mainly at KPSF. Elsewhere, VFR conditions are expected for the remainder of the period as high pressure builds across the Mid Atlantic Monday. Breezy west/northwest winds initially with gusts around 20-30 kts will gradually subside to around 5-15 kts tonight into Monday, and will back to the south/southwest late in the period.

Outlook...

Monday Night: Moderate Operational Impact. Chance of SN. Tuesday: High Operational Impact. Definite RA...SN. Tuesday Night: Moderate Operational Impact. Breezy. Chance of SN. Wednesday: Low Operational Impact. Breezy. NO SIG WX. Wednesday Night: No Operational Impact. NO SIG WX. Christmas Day: Low Operational Impact. NO SIG WX. Thursday Night: Moderate Operational Impact. Chance of SHRA. Friday: Moderate Operational Impact. Likely SHRA...SHSN.

ALY WATCHES/WARNINGS/ADVISORIES

CT...None. NY...Winter Weather Advisory until midnight EST tonight for NYZ038. MA...None. VT...None.